1. Field of the Invention
The present invention relates to a circuit substrate or particularly relates to a thermally conductive substrate employed for packaging power electronics, a manufacturing method therefor and a power module incorporating a thermally conductive substrate.
2. Description of the Related Art
In recent years, following improvement in the performance of electronic equipment and demand for miniaturization, it is desired to increase the density of a semiconductor and to improve the function thereof. Due to this, circuit substrates for packaging semiconductors are also desired to be small in size and high in density. As a result, design in which radiation of the circuit substrate is taken into consideration becomes important.
As circuit substrates exhibiting good radiating property, various types of circuit substrates have been developed conventionally. It is not, however, easy to maintain a circuit substrate to have good radiating property while holding down the price thereof.
As thermally conductive substrates to solve such disadvantages, there is conventionally known one disclosed by U.S. Pat. No. 6,060,150. The thermally conductive substrate disclosed therein is constituted by forming an insulator sheet having a sufficient thermally conductive filler to increase radiating property, lead frame and a radiation plate integrally with one another.
This substrate is manufactured as follows. A film is formed by mixing thermosetting resin having flexibility in an unhardened state with a thermally conductive filler (or an inorganic filler) to thereby produce an insulator sheet filled with the inorganic filler at high concentration for a thermally conductive substrate. Then, the insulator sheet, the lead frame and the radiation plate are built up with the insulator sheet put between the lead frame and the radiation sheet and the resultant buildup layers are heated and pressurized. As a result, the insulator sheet flows in the surfaces of the lead frame and hardened, thereby integrating the lead frame and the insulator sheet with each other. Further, the radiation plate adheres to the surface of the insulator sheet opposite to the surface to which the lead frame adheres.
Here, for the purpose of stabilizing the shapes of the lead frame and facilitating a processing for integrating the lead frame with the insulator sheet, all the end portions of the lead frame are coupled to a frame-shaped common outer peripheral portion. Due to this, the outside residues and outer peripheral portion of the lead frame on the thermally conductive substrate thus manufactured are removed except for the necessary parts of inside of the lead frame.
FIG. 13 shows the thermally conductive substrate thus manufactured. In case of the thermally conductive substrate shown in FIG. 13, the lead frame 800 protruding to the side surfaces of the thermally conductive substrate are bent perpendicularly (or in a direction orthogonal to substrate surface) so as to use the tip ends of the lead frame 800 as external lead electrodes.
The thermally conductive substrate stated above has disadvantages in that discharge tends to occur between the lead frame 800 and the radiation plate 802, thereby disadvantageously causing the damage of the substrate. The reason is as follows. To maintain the good thermal conductivity of the thermally conductive substrate, it is necessary to thin the insulator sheet 801. If so, however, the lead frame 800 and the radiation plate 802 become excessively adjacent each other and the creeping distance between the radiation plate 802 and the lead frame 800 cannot be sufficiently secured.
To prevent such a discharge phenomenon, it is proposed to secure a large creeping distance by locating the bent portions 800a of the lead frame 800 at a positions slightly inside of the end faces of the insulator sheet 801 in the sheet plane direction.
If so, however, the lead frame 800 is bent inside compared with the end faces of the substrate, with the result that a region on the thermally conductive substrate in which components can be actually mounted becomes small relatively to the substrate size.
Further, to bend the lead frame 800 toward the inside of the sheet plane of the insulator sheet 801 relatively to the end faces of the sheet 801, the lead frame 800 is required to be peeled off from the insulator sheet 801. This may probably damage the thermally conductive substrate.
To avoid such damage, it is proposed take the following measures. By providing steps on the insulator sheet 801 on portions (i.e., sheet end portions) of the sheet 801 on which the lead frame 800 is pulled out, the step portions of the lead frame 800 is exposed. By doing so, it is possible to prevent the thermally conductive substrate from being damaged without the need to peel off the bent portions 800a of the lead frame 800 from the insulator sheet 801.
To provide such steps on the insulator sheet 801, however, there is no avoiding making the shape of a metallic mold used to manufacture the substrate complex, which hampers cost reduction.
The main object of the present invention is, therefore, to provide a thermally conductive substrate and a manufacturing method thereof capable of securing a creeping distance between a lead frame and a radiation plate, locating the bent portions of the lead frame on the end faces of an insulator and thereby making the substrate small in size.
To obtain the above object, a thermally conductive substrate according to the present invention is characterized that a lead frame is provided on one surface of an insulator sheet and a radiation plate is provide on the other surface of the insulator sheet; a part of the lead frame extends to an end portion of the insulator sheet; and an end portion of the radiation plate located on and near the end portion of the insulator sheet to which the lead frame extends, is provided at a position away from the end portion of the insulator sheet inside of the insulator sheet in a plane direction of the insulator sheet. It is, therefore, possible to sufficiently secure a creeping distance from the lead frame to the radiation plate on the end portion of the insulator sheet to which the lead frame extends.
It is preferable that the end portion of the radiation plate away from the end portion of the insulator sheet inside of the insulator sheet in the plane direction of the insulator sheet is provided over an entire periphery of the radiation plate. By doing so, it is possible to secure the creeping distance and to manufacture the thermally conductive substrate relatively easily without complicating the structure of the radiation plate.
It is also preferable that a clearance between the end portion of the radiation plate located on and near the end portion of the insulator sheet, to which the lead frame extends, and the end portion of the insulator sheet is set to fall within a range of one to four times as large as a thickness of the insulator sheet. By so setting, it is possible to sufficiently secure the creeping distance and to prevent dielectric breakdown on the creeping surface of the substrate even if a high voltage is applied to the substrate.
It is further preferable that the radiation plate is arranged to be embedded in the insulator sheet while a surface of the radiation plate is exposed from the insulator sheet. By doing so, the end portion of the radiation plate along the thickness direction of the radiation plate is partly or entirely (i.e., the side surface of the radiation plate is) covered with the insulator sheet over the entire periphery of the radiation plate, whereby it is possible to further ensure preventing the dielectric breakdown of the thermally conductive substrate.
Furthermore, it is preferable that continuous steps along a direction crossing a shortest direction between the radiation plate and the lead frame are provided on the end portion of the insulator sheet, the end portion of the radiation plate arranged to be away from the end portion of the insulator sheet. By doing so, it is possible to further extend the creeping distance because of irregularities formed by the steps on the end portion of the insulator sheet.
Also, it is preferable that the insulator sheet contains an inorganic filler. If so, the radiation effect of the thermally conductive substrate is further improved.
Moreover, it is preferable that at least a part of a region along a side surface of the radiation plate in a thickness direction of the radiation plate is exposed from the insulator sheet over an entire periphery of the radiation plate and that an external radiation structure is plane-bonded to an outer surface of the radiation plate. By doing so, it is possible to sufficiently secure the creeping distance between the external radiation structure and the lead frame.
The thermally conductive substrate of the present invention can be manufactured by a method comprising a step of building up the lead frame on the one surface of the insulator sheet, building up the radiation plate on the other surface of the insulator sheet over an entire surface of the insulation sheet, and bonding the lead frame, the radiation plate and the insulator sheet to one another; and a step of removing an end portion of the radiation plate located on and near an end portion of the insulator sheet, to which the lead frame extends, up to a position away from the end portion of the insulator sheet inside of the insulator sheet in a plane direction of the insulator sheet. Here, the end portion of the radiation plate can be removed by cutting the end portion or by a photolithography step. In addition, a radiation plate having a split groove along a peripheral edge of a to-be-removed radiation plate region can be prepared as the radiation plate; and after the radiation plate is bonded to the insulator sheet, the to-be-removed radiation plate region can be divided from other radiation plate regions and removed along the split groove. In this case, it is possible to remove the end portion of the radiation plate relatively easily and surely.
Furthermore, another example of a method of manufacturing the thermally conductive substrate of the present invention may comprise the step of preparing, as the radiation plate, a case-added radiation plate, an end portion of the radiation plate corresponding to and near the end portion of the insulator sheet, to which the lead frame extends, being removed in advance, a case surrounding an entire periphery of the radiation plate being arranged outside of the radiation plate, and building up the lead frame on the one surface of the insulator sheet and the case-added radiation plate on the other surface of the radiation plate to bond the lead frame, the insulator sheet and the case-added radiation plate to one another; and removing the case from the insulator sheet.
Additionally, a power module according to the present invention comprises a thermally conductive substrate, an electronic component, a casing and sealing resin, characterized in that the thermally conductive substrate is constituted such that a lead frame is provided on one surface of an insulator sheet and a radiation plate is provide on the other surface of the insulator sheet, that a part of the lead frame extends to an end portion of the insulator sheet, and that an end portion of the radiation plate located on and near the end portion of the insulator sheet to which the lead frame extends, is provided at a position away from the end portion of the insulator sheet inside of the insulator sheet in a plane direction of the insulator sheet; the electronic component is packaged on one surface of the thermally conductive substrate; the casing is arranged to cover the thermally conductive substrate on which the electronic component is packaged; and the sealing resin is filled in an internal space of the casing and seals the internal space. By doing so, it is possible to sufficiently ensure the creeping distance from the lead frame to the radiation plate on the end portion of the insulator sheet to which the lead frame extends.